The present invention relates to micromachining, and more particularly, to an improved method for constructing suspended structures such as diaphragms and membranes.
A number of micromechanical devices include a bridge or diaphragm that is suspended over a silicon substrate. When the bridge is suspended over a substrate, the bridge is typically constructed by cutting holes in a layer that is to become the bridge and then etching the material under the bridge layer thereby leaving the bridge layer suspended. When the bridge covers a significant area, control of the undercutting, particularly the depth of the undercut, can become a significant problem.
There are two methods known to the prior art for controlling the etching of the material underlying the bridge layer. In the first method, the bridge layer is deposited on a sacrificial layer having a thickness that is equal to the height of the cavity to be left under the bridge. For example, a layer of silicon dioxide can be deposited or grown on a silicon substrate with the bridge layer deposited on the silicon dioxide. Holes are then cut in the bridge layer and the silicon dioxide layer removed utilizing an etchant that attacks the silicon dioxide much faster than the underlying silicon substrate and bridge layer. Unfortunately, there are a number of fabrication processes in which a sacrificial layer of sufficient thickness cannot be included without causing problems in the final structure or other parts of the fabrication process. In these situations, the undercut must extend into the underlying silicon substrate. Controlling the depth of the cut into the silicon substrate is difficult.
One method for controlling the undercutting of the bridge layer is taught in U.S. Pat. No. 4,895,616 to Higashi, et al. which describes the fabrication of a microbridge air flow sensor which has a sealed etched cavity beneath a silicon nitride diaphragm. The diaphragm in this system is constructed by forming a silicon nitride layer on the surface of a silicon substrate. A thin aluminum layer defines the top of the cavity. The aluminum layer is sandwiched between the silicon nitride layer and the silicon substrate. The diaphragm is created by creating a shallow cavity below the silicon nitride layer. This cavity is then extended into the underlying silicon substrate.
The aluminum sacrificial layer defines the area under the bridge layer that is to be removed. Since the aluminum can be selectively etched, a relatively small number of vias needs to be opened in the silicon nitride layer to provide sufficient access for an aluminum etchant solution to dissolve the layer leaving a narrow cavity under the silicon nitride layer. The bottom of the cavity can then be extended into the silicon substrate via a KOH etchant introduced into this cavity. This fabrication method allows a very large xe2x80x9cpitxe2x80x9d to be constructed under the diaphragm while minimizing the time of exposure of the diaphragm and any components thereon to the silicon etchant.
While this method represents a significant improvement over methods that do not utilize an aluminum sacrificial layer to define a narrow cavity that is later expanded, the method has one significant drawback. It is only applicable to fabrication processes that do not subject the aluminum layer to high temperatures. The aluminum layer is deposited before the membrane material and any components that are fabricated on the membrane are constructed. If any of the subsequent fabrication steps involves temperatures above the Sixe2x80x94Al eutectic point, the membrane integrity may be compromised. This limits processes based on aluminum sacrificial layers to temperatures of about 450xc2x0 C. and below. This constraint limits the use of this method. For example, the fabrication method taught in above cited patent cannot utilize the preferred silicon nitride deposition system (LPCVD) because this deposition system requires temperatures in excess of those permitted by the aluminum sacrificial layer.
Broadly, it is the object of the present invention to provide an improved method for creating suspended structures such as membranes and the like.
It is a further object of the present invention to provide an improved method of fabricating suspended structures that does not limit the temperatures that may be utilized in the fabrication process to be less than the Sixe2x80x94Al eutectic point.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention comprises a method for fabricating a suspended structure including a layer of membrane material over a substrate. The suspended structure overlies a cavity in the substrate. The method starts by generating a sacrificial layer comprising a first material that can withstand temperatures typically encountered in subsequent conventional semiconductor processing steps. In the preferred embodiment of the present invention, the bond between sacrificial layer and the underlying substrate must be capable of withstanding temperatures greater than the Sixe2x80x94Al eutectic point. A layer of membrane material is then deposited over the sacrificial layer. The membrane material comprises a second material different from the first material. An opening is introduced in the layer of membrane material thereby exposing the sacrificial layer. A first etchant is applied to the sacrificial layer through the opening until the sacrificial layer is removed leaving a portion of the cavity. The first etchant is chosen such that the first etchant removes the first material more rapidly than the second material. Finally, a second etchant is introduced into the cavity to expand the cavity. The second etchant is chosen such that the second etchant removes the substrate more rapidly than the second material. The first material is preferably PSG or thermal silicon dioxide.